The present invention relates to the analysis of a process stream of room temperature vulcanizable (RTV) silicone material to determine the content of a crosslinking agent therein through the use of continuous and automatic infrared laser spectroscopy.
Room temperature vulcanizable materials (RTV's) are silicone rubber compositions typically made up of a silanol-terminated linear diorganopolysiloxane polymer, one or more crosslinking agents, a catalyst, and a filler or other optional ingredients. In an acetoxy system of one-package or one-component RTV's for example, one of the primary crosslinkers is methyltriacetoxysilane. It is desirable to know the relative concentration of the crosslinker in the RTV composition because the amount of crosslinker directly affects the properties of the final product, i.e., the rate of cure and the viscosity of the cured product are dependent upon crosslinker content.
These compositions are prepared and stored in an anhydrous state and will cure to a silicone elastomer in the presence of atmospheric moisture. These products are utilized for a variety of purposes as, for example, formed-in-place gaskets and are widely used in many sealant applications. Silicone rubber compositions are quite adhesive to most substrates and exhibit outstanding resistance to ozone and ultraviolet rays as well as adverse weather conditions. Such compositions also have desirable tensile strength and modulus in many applications as well as being capable of performing effectively from very low temperatures such as -60.degree. F. to excessively high temperatures such as 300.degree. F. or more. Another advantage of one-component RTV compositions is their ability to be utilized directly at a job site without prior mixing.
RTV silicones are relatively viscous materials which are difficult to mix thoroughly. Typically, an RTV could exhibit a viscosity during its manufacture from about 75,000 centipoise to about 600,000 centipoise or more in its uncured state. It is therefore important that the crosslinker concentration be readily determined in order to provide a consistent final product.
Analysis methods and apparatus heretofore employed in the art were susceptible to several problems overcome by the present invention. For example, the crosslinker concentration could be determined by standard laboratory titration techniques, but such techniques are inherently non-continuous and must ordinarily be carried out by a skilled technician.
The methyltriacetoxysilane crosslinker mentioned above contains a carbonyl group which can be identified by titration. However, titration techniques also determine all other carbonyl-containing species in a given sample whereas the present invention provides a method and apparatus for selectively determining the active or effective crosslinker content in a given sample. The effective amount of crosslinker can be less than the total amount because of side reactions which could occur. For example, when methyltriacetoxysilane is used as a crosslinker, acetic acid is a volatile byproduct of the crosslinking reaction and a certain amount of this acid can be formed during manufacture, especially when conditions are not absolutely anhydrous. The method of the present invention can selectively determine the content of a particular constituent in a process stream such as the effective amount of catalyst while ignoring the presense of an interferring material such as acetic acid.
The carbonyl group in the methyltriacetoxysilane crosslinker absorb infrared radiation strongly at certain frequencies. An example of infrared spectroscopy utilizing a laser absorption spectrometer is disclosed in U.S. Pat. No. 3,856,406--Noble, issued Dec. 24, 1974, and assigned to the same assignee as the present invention, wherein an unknown gas sample is analyzed in an air pollution monitoring technique. U.S. Pat. No. 3,987,304--Kreuzer, issued Oct. 18, 1976, discloses infrared laser absorption spectroscopy of a gaseous stream such as the effluent stream of a retention time chromatograph. The Kreuzer apparatus utilizes an optoacoustic detecting means but does not teach analysis of an RTV process stream.
While lasers have heretofore been utilized for infrared absorption spectroscopy, materials which lend themselves more readily to such techniques are ordinarily analyzed. Until the present invention, it has not been possible to continuously analyze a room temperature vulcanizable silicone composition because of properties inherent to these materials.
One property of RTV which presents problems in the analysis of these compositions is the high viscosity of the material. The material also exhibits a tackiness that makes it difficult to obtain uniformly consistent samples. These properties of RTV make it nearly impossible to obtain on a continuous basis uniform thin samples required by standard methods of infrared analysis when it is diluted with a solvent which is different.
Accordingly, in order to continuously analyze an RTV process stream at a rate which substantially corresponds to the rate at which the material is being manufactured and moving in the stream, it is necessary to utilize a relatively much thicker sample of RTV. While a thicker sample overcomes the aforementioned problems concerning flow rate, it compounds a problem associated with infrared spectroscopy. Specifically, thick samples of RTV materials are relatively opaque with respect to infrared radiation at the frequencies which are useful for analysis, i.e., most of such radiation is absorbed by the material and little is left to pass through the sample for analytical purposes.
It has been suggested that conventional attenuated total reflectance laser spectrometers can be used to overcome the above-mentioned characteristic opacity of RTV materials; however, such an approach does not satisfy the requirement for continuous analysis. Nor do standard methods of infrared spectroscopy which utilize incandescent sources of infrared radiation or lasers with an intensity of about one milliwatt satisfy such requirements. Ordinarily, infrared spectroscopy is used to analyze materials such as a gas or a liquid which is relatively transparent to infrared. In such applications most of the infrared passes through the sample and only a small fraction thereof is absorbed by it. The small amount that is absorbed can be detected and analyzed. For RTV materials, however, over 98% of the infrared radiation is absorbed by the sample. In light of this, the present invention employs high intensity lasers and sensitive detecting means in order to determine the characteristic absorption of a constituent in the process stream under consideration.
One object of the present invention is to provide a novel method and apparatus for continuous and automatic analysis of a flowing RTV process stream for the content of a crosslinking agent or other constituent therein.
Another object of the present invention is to provide a novel method and apparatus employing a high intensity laser as the source of infrared radiation in order to overcome a number of problems previously encountered in the analysis of RTV.
Another object of the present invention is to provide a novel method and apparatus employing a high intensity source of radiation which obviates the need for a relatively thin sample to be analyzed, whereby analysis can be accomplished quickly and continuously.
An additional object of the present invention is to provide a new and improved apparatus which comprises a particular high intensity infrared radiation source and adapting the apparatus for effective operation in spite of the characteristic relative infrared opacity of RTV material.
Still another object of the present invention is to provide a novel method and apparatus such that the continuous analysis of an RTV composition takes place at a rate that corresponds substantially with the flow rate of the main portion of the RTV process stream from which the portion to be analyzed is taken, and does not affect the properties of the materials that have been analyzed.
These and other objects and advantages of the present invention will become better understood from a reading of the following specification and claims and consideration of the accompanying drawing.